In an internal combustion engine the greatest wear of the piston rings occurs at the free extremities (tips) or neighborhood thereof, by virtue of the greater contact pressure.
Described more specifically, the wear of the tips of the rings may be up to three times greater than that occurring in other regions of the part, the great focus of occurrence of the phenomena of spalling and peeling of the coating taking place at that point.
Diverse techniques have been developed having the objective of reducing this wear on the extremities of the piston rings, the basis/focus thereof, however, being the piston ring alone. The diverse techniques developed for cylinder liners in general relate to the reduction of friction.
A first development is revealed in the case of patent DE 102006057111, property of the applicant, relating to a piston liner wherein the roughness varies along the length in the sense of displacement of the piston within the interior thereof. More specifically, the portion adjacent to the maximum travel of the piston in the direction towards the head possesses a region having a first roughness and the central region of the liner, in terms of the travel of the piston, presents a second roughness, it being that on the intermediate portion of the working surface of the liner the roughness value exceeds that of the extremities. The focus of this document however resides solely in the specifications of the liner, without this being linked to the type of ring which will slide within the interior thereof, the distribution of peaks and valleys (Rpk, Rk and Rvk values) on the surface of the liner not being specified.
The patent document DE 102009010791 reveals a cylinder liner provided with a roughness value greater at the extremities in relation to the roughness value in the central region. However the process utilized for the obtainment of these structures leads to the incidence of recesses having greater and lesser depth (the depth varying considerably therebetween), diminishing the potential effectiveness of this solution as lubricating oil accumulation pockets. However, moreover, the focus of this document resides solely in the specifications of the liner without this being linked to the type of ring which will slide within the interior thereof.
The patent document DE 19605588 relates to a cylinder liner the internal surface whereof presents greater roughness values at the extremities in relation to the central portion. However in order for this result to be achieved the honing process having some technical parameters to treat the surface of the extremity regions is utilized, utilizing different parameters in the central region. Furthermore, the focus of this document resides solely in the specifications of the liner without this being linked to the type of ring which will slide within the interior thereof.
The patent document JP 2004/176556 defines a cylinder liner the angle between the recesses whereof is variable, being greater (angle α) at the extremity of the travel of the piston than on the central portion (angle γ). On the intermediate portions, between the central portion and each of the extremities, the angle (β) is intermediate.
Finally, patent document FR 2884889 relates to a cylinder liner the sliding surface whereof presents three roughness values, being (i) a first value S1, lower (less rough) in the extremity portion facing the head, in the central portion and in the opposed extremity, (ii) a second roughness value S2 of medium roughness applied between the extremity portion facing the head and the central portion and, finally, (iii) a third higher roughness value S3 applied between the central portion and the extremity portion opposed to that facing the head. Herein, also, the focus resides solely in the specifications of the liner without this being linked to the type of ring which will slide within the interior thereof.
Having the objective of reducing this wear the applicant has developed the present sliding assembly wherein the cylinder liner has an internal wall presenting variation in the roughness of the contact surface such that, considering the reciprocating motion of a piston within the interior of the liner, the central portion presents a roughness lower than that presented by the two limiting portions of the stroke of the piston (top dead center, TDC, and bottom dead center, BDC). In the present solution, whilst the roughness varies, the structure of the recesses and projections (‘valleys’ and ‘peaks’) defining the roughness is substantially uniform, permitting a greater study and a greater efficiency of the recesses in terms of the acting thereof as oil reservoirs.
In terms of the piston ring, this possesses a ceramic coating applied by the PVD (physical vapor deposition) process and this coating, taken together with the aforementioned characteristics of the contact surface of the liner, ensure a great resistance to wear.